FLASH cells, such as the one shown in plan in FIG. 1, (comprise twin gates 1 and 2, each with its own drain line, 11 and 12 respectively, and a shared source line 5 that is located between them. In order to minimize cell size while at the same time avoiding short circuiting the source line to the edges of the gates, a self-alignment procedure has been routinely used for source line formation. This procedure is a version of the SALICIDE (self-aligned silicide) process). Spacers were grown on the vertical side walls of the gate pedestals, following which metal was deposited over the entire structure. This metal was selected for its ability to readily form a silicide with any silicon with which it was in contact while at the same time not reacting with the material of the spacers (typically silicon oxide or silicon nitride). Then, when all unreacted metal was selectively etched away, the metal silicide was left in place, separated from the gate pedestal by the spacers. For the metals in question (typically titanium, cobalt, and nickel) the suicides are sufficiently good conductors that they may then be contacted with metal at some point removed from the gates without introducing significant series resistance (see for example drain contacts 3 and 4 in the figure).
When FLASH cells were first introduced, dielectric isolation between devices was accomplished using the LOCOS (Local Oxidation of Silicon) process. As its name implies, LOCOS involves oxidizing the silicon surface (in the presence of masks) which results in an oxide-silicon interface that slopes inwards away from the surface giving it a shape commonly referred to as `bird's beak`. In this situation, it was reasonable to etch the control gates before forming the self-aligned source line. As a result, oxide in the LOCOS regions between the gates was removed before the silicon nitride spacers were laid down causing the spacers to extend into them. This did not cause a problem because the LOCOS shape is not as sharp as an STI edge.
Because of its improved utilization of real estate on the chip surface, shallow trench isolation (STI) has been gradually replacing LOCOS. Seeing no reason to change other parts of the manufacturing process, workers in the field have continued to form the self-aligned source contacts according to the process described above. This is best illustrated by referring to FIGS. 2A, B, and C which are cross-sections taken through A--A, B--B, and C--C respectively in FIG. 1.
In FIG. 2A each gate is seen to be a compound structure of a control gate 1 or 2 (as seen in FIG. 1) and a floating gate 21 or 22. A conventional layer of gate oxide 23 is present between the floating gates and the surface of integrated circuit 25, while layer 29 of ONO (oxide, nitride, oxide) separates the floating gate from the control gate. The ONO acts as an isolation layer to separate the control gate from the floating gate.
In FIG. 2B, we see a cross-section made at a location somewhat removed from the main gate structure, showing that the control gate (or, more precisely, its extension in the form of the polysilicon word lines 6 and 7) rests on silicon oxide 24 which fills the STI trenches except for trench 26 from which the oxide has been removed in order to allow the still-to-be-formed source line to make contact with the silicon 25.
FIG. 2C is a cross-section taken through the space between the gates showing oxide free region 26 (as seen in FIG. 2B) as well as its counterpart 27 located on the other side of the inter-gate space.
FIGS. 3A-C show the structure after silicon nitride spacers 33 have been formed. While the spacers have their normal appearance in FIG. 3A, it can be seen that in FIGS. 3B and 3C the spacers extend beyond the control gates 6 and 7 down into the trenches. In the older LOCOS process, spacer material would be removed from the trench side-walls as part of the spacer formation process itself, this removal occurring because the sidewalls were not vertical, but, in STI, the sidewalls are very nearly vertical so that the process can no longer distinguish between material on the sidewalls of the gates and material on the sidewalls of the trenches. The consequences of this are that when the next step in the process is executed (namely self-alignment through SALICIDE) the residual stringers of silicon nitride on the trench sidewalls interfere with formation of conductive silicide, leading to open circuiting in, or high resistance of, the source line (5 in FIG. 1). Another disadvantage of this process is that additional etching of the spacers in order to reduce their thickness (thereby increasing the width of the source line) is not desirable because an additional mask would be needed to protect other silicon nitride parts contained within the structure.
The present invention shows how the above problems may be avoided. We have performed a routine search of the prior art but did not find any patents that teach a solution similar to that of the present invention. Several references of interest were, however, encountered. For an example of forming self aligned contacts when LOCOS is used, see Liang et al. (U.S. Pat. No. 5,665,623). Hong (U.S. Pat. No. 5,703,387) uses shallow trench isolation but implements the self alignment portion of his process with oxide, spacers. Hsu et al. (U.S. Pat. No. 5,552,331) are concerned with a process involving several different device types and deal with this by using two different sets of spacers, formed to slightly different dimensions. Chen et al. (U.S. Pat. No. 5,751,040) best represent the STI based prior art that has been discussed above--they allow the spacers to form on the side walls of the trenches.